I'm not sure why you chose to only read the final quarter of my post, but I think you'll find the rest of it answers pretty much every question you asked there.
To explain: If the CDC is announcing this, it's because they've determined that fully vaccinated people do not present a significant enough risk of being infected or transmitting the disease to warrant wearing a mask. I don't doubt that, especially as vaccination rates climb. That's not the problem.
The problem is that countless idiots are going to use this to justify eliminating mask mandates and/or lie about their vaccination status so they don't have to wear a mask. Unless mask mandates are kept in place, that's going to result in rising case numbers among that population, diminishing the effectiveness of existing vaccines and creating a breeding ground for variants that may get through the existing vaccines. This is also going to mean that, if a variant emerges that isn't prevented by the vaccines, the vaccinated population is going to be particularly vulnerable to that because they aren't masked, and especially so because getting them to wear masks again after telling them they don't have to is going to be hard.
Don't get me wrong, it's good that the science is supporting this, because that's a major milestone in the fight against covid. I also don't doubt that the CDC has analyzed the data properly. I just don't trust the general public to not screw this up.
I mean if the government really wanted to ensure only those fully vaccinated people get this they could create an ID or something that shows that they have been fully vaccinated. It would be quite an expense creating a special ID just for that though, so I doubt it happens.